Isotope Effects in Methoxy Radical Chemistry

Suny College Of Environmental Science And Forestry, Syracuse NY

Investigators

Abstract

This project has three goals: 1) Experimentally determine the branching ratio for abstraction of hydrogen atoms (H) versus deuterium atoms (D) from mono-deuterated methoxy radicals (CH2DO*) in their reaction with molecular oxygen (O2) as a function of temperature; 2) Experimentally determine absolute rate constants for CH3O* and CD3O* reacting with O2 as a function of temperature; 3) Compute rate constants for methoxy + O2 reactions, including tunneling and variational effects, and extend calculations to larger radicals. Two experimental approaches will be used: Fourier Transform Infrared (FTIR) spectroscopy in the reaction chamber of collaborators at the National Center for Atmospheric Research (NCAR) will be used to determine the branching ratio for production of deuterated versus normal formaladhyde in the CH2DO* + O2 reaction. Rate constant ratios for reaction of CH3O* and CD3O* radical with O2 (kO2) and NO2 (kNO2) will also be determined at NCAR from product yields. Experiments at NCAR will be carried out by one of the Principal Investigator's (PI) graduate students under the direct supervision of senior scientists at NCAR. Direct measurements of kNO2 at the PI's lab (SUNY-ESF) will be carried out using laser flash photolysis to generate radicals and laser-induced fluorescence for time-resolved detection. The combination of kNO2/kO2 from NCAR with kNO2 from SUNY-ESF will enable determination of kO2(T) for both CH3O* and CD3O*. Direct measurements of kO2 at SUNY-ESF will validate the combined results. An understanding of the isotope effects in these reactions will be achieved via high-level quantum calculations coupled to cutting-edge algorithms for statistical rate theory and multi-dimensional tunneling calculations. Calculations will be extended to larger alkoxy radicals. This research will be the first to determine kO2 at temperatures less than 298 K for methoxy radical. It will also be the first temperature-dependent determination of branching ratio for production of normal and deuterated formaldehyde in the CH2DO* + O2 reaction, and only the second study of this branching ratio. The calculations will provide benchmarks for reliable calculations of kO2(T) for a diverse range of alkoxy radicals of tropospheric interest. By enhancing other researchers' abilities to calculate reliable values of kO2(T), this research will lead to a much better understanding, not just of alkoxy radical chemistry, but also of the overall mechanisms of degradation of many classes of volatile organic compounds (VOCs). This will help improve representations of VOC degradation processes in models of air pollution and global tropospheric chemistry, contributing to more effective ozone abatement plans and better modeling of climate-chemistry feedbacks. The results will also constrain the mechanism of deuterium enrichment of molecular hydrogen in the atmosphere, and improve our understanding of the atmospheric budget of molecular hydrogen. This will help to understand the potential impacts of a hydrogen economy on microbial communities, stratospheric and local ozone, and the abundance of greenhouse gases. Two graduate students and several undergraduates working on this project will grow intellectually and professionally, and gain advanced technical knowledge of diverse experimental and computational methods for kinetics.